Once-through steam generator

ABSTRACT

A once-through steam generator comprises an economizer, an evaporator and a superheater connected in series one below the other in a boiler pass, and are acted upon by a hot gas mass flow flowing in the opposite direction. In order to prevent thermal shock during the introduction of cold feed water when starting the steam generator from a hot state, i.e., with an economizer, evaporator and superheater which are empty and heated to the exhaust gas temperature, a high-temperature starter is connected upstream of the economizer and is arranged above the latter in the boiler pass. The starter acts as a heat buffer so that the economizer, evaporator and superheater are first acted upon by superheated steam and are continuously cooled to their normal operating temperature. The starter is constructed in such a way that its portion which first comes into contact with the cold feed water does not support the steam pressure so that it need only absorb thermal stresses. In normal operation, the starter works as a preheating stage for the economizer.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to steam generators and, inparticular, to a new and useful once-through steam generator having ahigh-temperature heat accumulator or starter connected upstream of aneconomizer or evaporator of the steam generator.

Once-through steam generators are used, for example, in combined cyclepower stations as waste heat recovery equipment of gas turbines. Suchsteam generators, which are constructed as once-through boilers, makeuse of hot exhaust gases of the gas turbines for the purpose ofgenerating additional steam. These exhaust gases generally have atemperature of over 500° C. They can be operated for short periods oftime in order to cover peak current requirements.

Known steam generators of this type, for example, as disclosed inETV-Register 45/86, page 59, or Power, April 1985, page 118, consistsubstantially of an economizer, an evaporator, which is connecteddownstream, and a superheater connected to the evaporator, which arearranged one above the other in a boiler pass. During operation, theexhaust gas mass flow, which is still hot, is conducted through theboiler pass, first through the superheater, then through the evaporator,and lastly through the economizer. The feed water entering theeconomizer is preheated therein, then arrives at the evaporator where itis evaporated to form wet steam. In the superheater, the wet steam isfinally converted to superheated steam and is then fed to a steamturbine. An outlet header is generally included between the evaporatorand superheater and between the superheater and the steam turbine.

When the generated steam is not required, e.g., during times of lowcurrent consumption, the steam generator is shut down. In systems havingan exhaust gas diverter, after interrupting the feed-water feed, theexhaust gases are by-passed from the still running gas turbine throughthe exhaust gas diverter duct to a chimney. The boiler then cools off.

If no exhaust gas diverter is available, which may be the case inonce-through boilers, the water content still present in the boiler whenshut down is evaporated by the exhaust gases of the gas turbine whichcontinue to flow through the boiler. The economizer, evaporator andsuperheater then become empty and their tubes become heated to exhaustgas temperature. In order to put this steam generator which is now in aheated state into operation again, the economizer, the evaporator andthe superheater must first be cooled-off to their normal operatingtemperature by means of the feed water, and the water supply of theboiler must be restored.

When starting up the cooled off steam generator with an exhaust gasdiverter, a diverter damper remains open so that the hot exhaust gasmass flow is guided into the atmosphere in a circuitous manner aroundthe economizer, evaporator and superheater so as to be completelyunused. During this start-up phase, the economizer and evaporator arerefilled with feed water, after which the gas diverter is closed and thehot exhaust gas mass flow is again guided through the boiler pass. To agreat extent, this prevents temperature shock, such as would occur bythe entry of cold feed water into the boiler which is heated to theexhaust gas temperature.

The use of the gas diverter is costly and maintenance-intensive. Thediverter dampers for opening and closing the diverter duct which havecorrespondingly large dimensions and which block and open the path ofthe exhaust gas through the boiler, involve considerable problems withrespect to tightness. Because of the different local temperatures andthe enormous size of the dampers, the dampers often warp and then nolonger close so as to be tight. Also, the control and blocking devicesfor the diverter dampers are costly. Since the diverter duct mustconduct the entire volume of exhaust gas around the boiler, it requiresadditional space for the diverter ducts and increases the costs of theheat recovery system.

If no exhaust gas diverter is provided, the empty steam generator isheated at the exhaust gas temperature after the "evaporization" of allwater from the boiler tubes. A disadvantage in these steam generators isthat, when starting up the hot steam generator, that is when refillingthe boiler with feed water, there are great differences in temperaturebetween the feed water, the economizer and the evaporator tubes whichproduce large thermal stresses.

SUMMARY OF THE INVENTION

Taking the foregoing as a point of departure, an object of the presentinvention is to construct a once-through steam generator which can bestarted up from the heated state without undue thermal stresses whichreduce the life of the steam generator tubes.

Accordingly, another object of the present invention is to provide aonce-through steam generator which comprises a boiler pass for confininga hot gas mass flow, the boiler containing an economizer, evaporator andsuperheater connected in series along the flow path with the gas massflow flowing in a direction from the superheater toward the evaporatorand economizer conducting water which is converted to steam in anopposite direction, and a high temperature starter connected to theeconomizer, upstream of the economizer in the direction of water flowand downstream of the economizer in the hot gas mass flow direction,thus providing additional heating surfaces in the flow path for heatingthe feed water for starting up the operation of the economizer,evaporator and super-heater.

The solution, according to the invention, is simple in terms ofconstruction and protects the economizer, evaporator and superheaterfrom temperature shock in an effective manner. When starting up theempty, hot steam generator, the hot starter which forms ahigh-temperature heat accumulator, which is connected upstream of theeconomizer, forms a thermal buffer which prevents cold feed water fromcoming into contact with hot tubes of the economizer, evaporator andsuperheater, previously heated by means of the gas turbine exhaust gasflow.

The starter which is provided for such a purpose absorbs this shockcaused by the feed water, which could be damaging to the economizer,evaporator and superheater, so that the tubes of the heating surfacesarranged downstream of the starter can cool off to the operatingtemperature slowly and smoothly without temperature shock and withoutthe occurrence of high thermal stresses which reduce tube life.

In normal operation or when starting the boiler from a cold state, thestarter acts as an additional water preheater and, in so doing, does notimpede the generation of steam. Therefore, any means for controlling thewater flow to the starter can be dispensed with.

When starting the empty steam generator from the hot state byintroducing feed water, this water, which is still cold is firsttransformed into superheated steam in the starter which has been heatedto the exhaust gas temperature during the preceding dry operation of theboiler. Thus, only this superheated steam initially reaches the tubes ofthe economizer, evaporator and superheater which are likewise at theexhaust gas temperature. To the extent that the starter releases theaccumulated heat and thereby cools off, super-heated steam at a lowertemperature flows into the economizer from the starter, which is thenfollowed by saturated and later, wet steam, and finally by hot water. Inthis way, the economizer, like the evaporator and superheater connecteddownstream thereof, cool off slowly from the exhaust gas temperature tothe normal operating temperature without temperature shock.

This gradual cooling from high temperature, or enthalpy at the starteroutlet, as well as the initially moderate heat transfer rate of steam,lessen the intensity of the cool off of the economizer and evaporatortubes and the adjoining headers. The heating surfaces of the steamgenerator arranged downstream of the starter are thus not subjected to atemperature shock, but rather undergo a continuous cooling. Thetemperature drop at the economizer inlet, which is delayed by means ofthe starter, is slowed down further by means of the heat stored in theeconomizer so that the flow of steam and later water cools the adjacentevaporator tubes with even smaller temperature transients. The greaterthe distance from the starter inlet, the lower the local temperaturetransients in the tubes.

A constructional form of the starter which is simple in terms ofconstruction and particularly favorable with respect to occurringthermal stresses comprises one or more heat accumulating elements whichare constructed from a plurality of interpenetrating coaxially arrangedtubes. The innermost tube receives feed water. An opposite open end ofthe innermost tube opens into the next coaxial tube. The water thenreturns outside the innermost tube through the further coaxial tube andis either discharged at an opposite end of the next coaxial tube oragain returned into a still further outer coaxial tube. The internaltubes provide a large heat-accumulating heat exchange surface and havethe advantage, with the exception of the outermost tube, that they areonly heat-loaded, i.e., subjected to heat, and not pressure-loaded.Since the feeding of feed water is effected through the innermost tube,where the greatest thermal stresses occur, the possible tubedeterioration, such as cracking in the tube wall, for example, does notresult in impairment of the functioning of the starter. The advantage ofthe coaxial tube arrangement consists in the possibility of free thermalexpansion of the internal tubes which are located in the respectivesurrounding tube with a clearance so as to be freely movable at theother end.

The outermost starter tube which is heated from the outside by means ofexhaust gases is initially cooled by means of steam arriving from theinternal tubes before it comes into contact with the feed water. Sincethe steam pressure in the boiler rises slowly, this outer tube issubstantially subjected to stresses due to the steam pressure only afterthe heat release of the starter is terminated. The starter can comprisea plurality of over two coaxially arranged interpenetrating tubes, butit is also possible for only two coaxial tubes which extend along theentire cross section of the starter tubes to form several loops, forexample.

It is particularly advantageous if the internal tubes of the starterwhich are not exposed to a pressure difference are constructed so as tohave thin walls. Since the thin-walled tubes are heated substantiallymore rapidly than thick-walled tubes, the thermal stresses occurring inthe tube wall, because of the differences in temperature during startingare smaller, so that thermal expansion can be effected approximatelysimultaneously in the entire tube wall, which prevents cracking in theprotective layer of the tubes and increases the life of the inner tubes.

The construction of such a starter from individual starter elementscomprising at least two or more interpenetrating pipes has provenparticularly advantageous. According to the required heat capacity, aplurality of these starter elements can then be connected in paralleland arranged above the economizer next to one another and/or one abovethe other in the boiler pass. Construction of the invention in whichthree tubes are coaxially interpenetrated with each other has provenparticularly advantageous This construction offers a relatively largeheat transfer surface with the possibility of respective free expansionof the two internal tubes while the space requirement remains moderate.Such a starter element is relatively simple to manufacture and thereforehas a low production cost.

Another construction of the high-temperature starter according to thepresent invention comprises a hollow casing through which feed water canflow and which contains an internal tube which is filled with heataccumulating bodies which permit water to flow around them. In thisform, the starter capacity is substantially formed by the heataccumulating bodies. The greatest part of the heat exchange surface,namely the surface of the heat accumulating bodies, lies in the internaltube. The bodies exposed to steam pressure from outside are looselyarranged so as to be freely movable. The heat accumulating bodies cantherefore expand and contract freely. This construction also offers theadvantage that the heat accumulating capacity of the starter can bevaried in a simple manner by means of the size and material of the heataccumulating bodies and is accordingly adaptable to given localconditions. The internal tube prevents contact between the water whichis fed into the internal tube and the casing supporting the steampressure. Superheated steam, wet steam and water successively flowthrough the annular space between the hollow casing and the internaltube.

For reasons relating to strength, a cylindrical shape for the hollowcasing is particularly suitable, since the thermal stresses occurringwithin such a hollow casing are relatively small. In addition, acylindrical hollow casing ensures a favorable flow characteristic withlow flow resistance.

Balls have proven particularly suitable as the heat accumulating bodies,since they do not tend to become jammed with one another and thus canmove relative to one another during thermal expansion. In addition, feedwater may flow around the balls on all sides so that a large heattransfer surface which can be exploited in practice is formed. This heattransfer surface cannot deteriorate in an uncontrollable manner by meansof surfaces adhering together as is the case, for example, in bodieswith plane surfaces.

It is also advantageous according to the invention to construct thestarter from several elements which can be prefabricated in series in aplant and connected in parallel in a determined quantity according tothe boiler dimensions and capacity of the system.

A construction of the invention with a hollow casing with filling whichcan be electrically heated, is advantageous for outfitting alreadyexisting systems with the starter of the invention. In thisconstruction, the hollow casing filling is electrically heatable so thatthe starter can also be arranged outside the boiler. Then, in order tostart the hot steam generator, the starter is first electricallypreheated, after which the feed water is admitted to the starter and tothe heat exchanger surface connected downstream. In this instance, also,the feed water is preheated inside the starter in the same way asdescribed above, so that temperature shock of the economizer, evaporatorand superheater is prevented.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objectives attained by its use,reference should be had to the drawings and descriptive matter in whichthere are illustrated and described the preferred embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a side view of the inventive arrangement for a starter in aonce-through boiler;

FIG. 2 is a cross-sectional view of the arrangement according to FIG. 1;

FIG. 3 is a longitudinal sectional view of a starter element of theinvention;

FIG. 4 is a sectional view along section line IV--IV of FIG. 3;

FIG. 5 is a schematic view of the construction of a once-through steamgenerator of the invention;

FIG. 6 shows another construction of a starter element of the inventionwith a hollow casing; and

FIG. 7 shows an arrangement of an electrically heatable starter of theinvention which is outside the boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The once-through boiler shown in FIGS. 1 to 5 substantially comprises aboiler pass 1 in which a high temperature starter 2, an economizer 3, anevaporator 4 and a superheater 5 are arranged one above the other, asseen along the longitudinal axis of the boiler pass. As shownschematically in FIG. 5, the individual structural components 2-5 arearranged inside the boiler pass 1 as tube bundles. During operation, ahot gas mass flow 6 flows around the components 2-5 from the bottom tothe top, wherein the hot gas mass flow 6 transforms water fed into thesteam generator at 7 into superheated steam at 10.

The admission of feed water into the boiler 1 is effected from thefeed-water inlet header 7. From the feed-water header 7 and over athrottle 8 which is provided to insure uniform water distribution to theparallel starter elements, the water first arrives at the top into theheating surfaces located in the boiler pass 1, that is, into the starter2. From the starter 2, the water flows through connecting tubes to theeconomizer 3 connected downstream thereof. The outlet of economizer 3 isconnected to the evaporator 4 located below it. The evaporator 4 isconnected to the superheater 5 located below that by means of an outletheader 9. The outlet of the superheater 5 opens into an outlet header 10which is connected via steam piping with a steam turbine (not shown).The feed-water and steam flow 11 runs from top to bottom of the boilerpass 1, wherein the hot gas mass flow 6 flows in the opposite direction,from bottom to top, through the boiler pass 1 and, in so doing, convertsthe feed water in the boiler components 2-5 to superheated steam.

The starter 12 (FIGS. 1 and 2) is arranged above the economizer 3 in theboiler pass 1 transversely relative to the boiler axis so as to beparallel to one another. The elements 12 are arranged in the water flowin parallel, their construction can be seen particularly from FIGS. 3and 4.

Each starter element 12 consists of a plurality of coaxiallyinterpenetrating tubes 13, 14 and 15. Three are used in thisconstruction. The feed water 11 coming out of the feed water header orinlet 7 flows through the innermost tube 13 into the starter element 12.This innermost tube 13 ends at a distance prior to the end of thesurrounding middle tube 14. This end of tube 14 is closed. The middletube 14, which is arranged to have a clearance between the inner tube 13and the outer tube 15, is open at the side into which the inner tube 13is inserted into the tube 14, and ends at a distance prior to an end ofthe outer tube 15. The outer tube 15 is closed at its two ends andincludes an outlet 16 near one end as seen in FIG. 3, for the flow 11.Element 12 is connected to economizer 3. In boilers without economizers,outlet 16 is connected to the evaporator 4. The interior tubes 13 and 14are constructed so as to be thin-walled since they are supposed toabsorb only thermal shock stresses, without being exposed to steampressure differences. Since the interior tubes 13 and 14 each have afree end, they can expand and contract freely without the occurrence ofmechanical stresses which usually arise in clamped-in tubes. A crack inone of the inner tubes 13, 14 also does not impair the effectiveness orstrength of the starter element 12 in any way.

According to FIGS. 1, 2 and 5, a starter element 12 is connected inparallel upstream of each of the tubes of the economizer 3. However, itis possible to connect two or more elements 12 in tandem in order toincrease the thermal shock absorbing effect. These starter elements 12are constructed as constructional units and can be combined to formstarters of different thermal capacities according to the building blockprinciple.

FIG. 6 shows another construction of a starter element 12', which, whenconnected in parallel and in a plurality above the economizer 3, can beused in the same way as the element 12 of the starter 2. The starterelement 12', comprises a cylindrical hollow casing 17 and an internaltube 23. At one end, element 12', comprises an inlet opening 18 and anoutlet opening 19. The inlet opening 18 is connected via pipes to thefeed-water header 7 and the outlet tube 19 is connected via tubes to theeconomizer 3. Numerous heat accumulating bodies 20 in the form of ballsare inserted inside the hollow internal tube 23 so as to loosely contacteach other and are secured in the vicinity of the end of the tube 23 onthe outflow side by means of a retaining screen 21. In operation, thefeed-water flow may flow around the heat accumulating bodies 20 onvirtually all sides so that a very high heat transfer surface isobtained when such starter elements 12', are used. The object of theinternal tube 23 is to prevent the direct contact of water flowing intothe element 12', via the inlet opening 18 with the external tube 17.

The aforementioned once-through boiler, which is equipped with astarter, works in the following manner: When the boiler is started froma cold state, feed water is fed to the boiler tubes which are laterheated by means of the introduced hot gas mass flow 6 until the systemis at operating temperature. Therefore, starting of the boiler from thecold state is effected as in conventional once-through boiler, whereinthe starter 2 has no particular function, but rather, only serves topreheat water before the economizer 3. Thus the starter 2 works as anadditional water preheating stage in normal operation.

The feed water enters the boiler 1 from the top through the throttle 8from the feed-water header 7 arriving first in the starter 2, which isthen working as a water preheater, and then arrives in the economizer 3,which is arranged below the accumulator 2, and to which the evaporator 4located below this is connected, the feed water being evaporated intosaturated steam in the evaporator 4. The saturated steam then arrives inthe outlet header 9 which is connected to the superheater 5 in which thesaturated steam is converted to superheated steam which is then fed tothe steam turbine via the outlet header 10. The transformation of thefeed water flowing into the boiler into superheated steam is effected bymeans of the hot gas mass flow 6 which is guided through the boiler pass1 from the bottom to the top in an opposite flow direction and which hasan inlet temperature of 300° C. to 600° C., for example. This hot gasmass flow 6 heats the tubes located in the boiler pass and, accordingly,the feed water. Because of the heat exchange between the gas and thewater, different regions occur in the vertical direction of the boilerpass 1, i.e., from top to bottom, a water preheating region, anevaporation region and a superheating region (corresponding to theeconomizer 3, the evaporator 4 and the superheater 5) which have bordersthat can migrate relative to one another.

When starting the steam generator from the hot state, the boiler pass 1and the tube system located therein consisting of starter 2, economizer3, evaporator 4 and superheater 5, are heated to the temperature of theexhaust gas flow 6, since the feed-water feed is interrupted and thefeed water contained in the boiler is completely evaporated. In order torestart the hot boiler, feed water is admitted directly to the boilerwhich is outfitted with the starter 2, while the exhaust gas flow 6continues to flow through the boiler pass 1. The cold feed water whichis introduced now first arrives in the starter 2. The feed waterarriving in the hot starter 2 is evaporated and further heated over ashort distance because of the great temperature difference and aconsiderable increase in volume takes place causing a steam pressureincrease to occur. In the starter 2 which is shown in FIGS. 1 to 5 andwhich is constructed from coaxial tubes 13, 14, 15, the greatesttemperature shock is effected at the inner tube 13 which, like themiddle tube 14, need not support any further considerable forces sincealmost equal pressure prevails at both sides of the tube wall. The tubewall of the outer tube 15 indeed must absorb stresses due to steampressure, but is only exposed to the temperature transients that havealready been reduced beforehand in the internal tubes. Thus, the tubes13, 14, 15 of the starter are subjected exclusively to stresses which donot lead to an impairment of their functioning.

Thus, from the starter 2, first superheated steam, then wet steam andfinally preheated feed water arrive in the economizer 3 and are thenfurther heated in the evaporator 4 and finally arrive in the form ofsteam in the superheater 5. In this way, the economizer 3, evaporator 4and superheater 5 are not subjected to a temperature shock, but ratherare continuously heated to operating temperature slowly and,accordingly, in a manner which is sparing of tube material.

Subsequently, since cold water is being fed into the starter 2continuously, the point of transition from water to steam is displacedin the longitudinal direction of the inner tube 13 and finally alsoalong the longitudinal direction of the middle tube 14. Due to thecooling effect, this transition migrates slowly through the starteruntil arriving at the outer tube 15 and, finally, into the area of theeconomizer 3 so that, after the startup, a normal operating temperatureis adjusted in which the heated preheating zone advances until theeconomizer outlet and from there makes the transition into steam.

The starter 2 which is described with the aid of FIG. 6 and compriseselements 12, acts in the same manner as described above. In thisconstruction, the hollow casing 17 is loaded by means of steam pressurewhich occurs during sudden evaporation of the feed water, but does notdirectly contact the feed water due to the internal tube 23. Rather, itis first cooled by means of the superheated steam forming at the heataccumulating balls 20. The heat accumulating balls 20 form the firstcontact surface with the feed water. Because of their high thermalcapacity and their large effective heat exchange surface, the wall ofthe hollow casing 17 is effectively protected from a sudden shock causedby cold water. The heat accumulating balls 20 are not exposed to anytensile stresses due to steam pressure, but they must absorb thermalstresses exclusively, which is why they are arranged so as to be looselyarranged within the internal tube.

In the arrangement shown in FIG. 7, the starter 2 is arranged outsidethe boiler pass 1. This starter 2 is constructed from two starterelements 12" which are connected in parallel and constructed like thestarter element 12' described with the aid of FIG. 6, but which comprisean electric heating means which is supplied with current via connections22. The heat accumulating balls 20 arranged inside the starter elementsof FIG. 7 can be heated by means of the electric heating means.

When starting the steam generator from the hot state, the starterelements 12" lying outside the boiler pass 1 and connected upstream ofthe economizer 3 are preheated to the desired temperature, after whichthe heating current is turned off and the feed water is admitted. Thestarter elements 12" then act as heat buffers in the same way aspreviously described and, accordingly, prevent excessively high thermalstresses, particularly in the region of the economizer 3.

As shown in the construction of FIG. 7, already existing systems can beoutfitted subsequently with a starter which can be arranged in thevicinity of the boiler and is heatable by means of electricity or insome other manner.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the inventiveprinciples, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A once-through steam generator comprising: aboiler pass defining a flow path for a hot gas mass flow flowing in afirst direction; an economizer, evaporator and superheater connected inseries in said boiler pass for leading a flow of water whichprogressively evaporates into superheated steam flowing in saideconomizer, evaporator and superheater, in a second direction which isopposite to said first direction; and starter means acting as ahigh-temperature heat accumulator and connected to said economizerupstream of said economizer in said second direction for preheating coldwater before it is supplied to said economizer for hot startup of saideconomizer, evaporator and superheater without thermal shock thereon,after said starter, economizer, evaporator and superheater have beenheated to the temperature of the hot gas mass flow.
 2. A steam generatoraccording to claim 1, wherein said starter means comprises a starterelement having at least one inner member for initially receiving asupply of cold water and for initially converting the cold water intosteam during an initial phase of the startup, said inner member beingconstructed so as to be relieved of stresses due to steam pressure andso as to receive thermal stresses, said starter means including an outermember enclosing said inner member and absorbing tensile stresses due tosteam pressure.
 3. A steam generator according to claim 2, wherein atleast one starter element comprises a plurality of coaxial tubes oneinside the other, each tube being attached at only one end for freethermal expansion and being made of relatively thin-walled material,said starter element comprising an outer tube made of relatively heaviermaterial enclosing said coaxial tubes and extending coaxially aroundsaid coaxial tubes in said flow path.
 4. A steam generator according toclaim 2, wherein said at least one inner member comprises an inner tubefilled with a multiplicity of heat accumulating bodies which are looselyarranged in said inner tube, and an outer hollow casing enclosing saidinner tube.
 5. A steam generator according to claim 1, wherein saidstarter means comprises a plurality of coaxially arranged tubes (13, 14,15) which are arranged one inside the other at a distance relative toone another in such a way that an open end of an inner tube (13, 14)ends at a distance prior to an end of a surrounding tube (14, 15) whichis closed, and wherein the outermost tube (15) is connected to saideconomizer (3) and the innermost tube (13) is connected to a source offeed water.
 6. A steam generator according to claim 5, wherein saidinner tubes (13, 14) are free of stress due to steam pressure and areconstructed so as to have thin walls relative to said outermost tube. 7.A steam generator according to claim 1, wherein said starter meanscomprises three tubes (13, 14, 15) which are arranged one inside theother and which together form a starter element (12), a plurality ofsuch starter elements (12) being connected in parallel to form saidstarter means.
 8. A steam generator according to claim 1, wherein saidstarter means (2) comprises a hollow casing (17) through which feedwater flows and which is filled with heat accumulating bodies (20) whichare arranged within the hollow casing (17) so as to be loosely arrangedand so that water and steam may flow around them.
 9. A steam generatoraccording to claim 8, wherein said hollow casing is constructed so as tobe cylindrical and is arranged transversely relative to said hot gasmass flow passage.
 10. A steam generator according to claim 8, whereinballs are provided as said heat accumulating bodies.
 11. A steamgenerator according to claim 1, wherein a hollow casing (17) which isfilled with heat accumulating bodies forms a starter element (12'), anda plurality of said starter elements being connected in parallel to formsaid starter means.
 12. A once-through steam generator comprising: aboiler pass defining a first flow path for a hot gas mass flow flowingin a first direction; an economizer, evaporator and superheaterconnected in series in said boiler pass for leading a flow of waterwhich progressively evaporates into superheated steam flowing in saideconomizer, evaporator and superheater forming a second flow pathextending into said boiler pass and within said boiler pass flowing in asecond direction which is opposite to said first direction; and startermeans acting as a high-temperature heat accumulator and connected tosaid economizer in said second flow path up-stream of said economizerfor preheating cold water before it is supplied to said economizer forhot startup of said economizer, evaporator and superheater withoutthermal shock thereon, said starter means (2) comprises a hollow casing(17) through which feed water flows and which is filled with heataccumulating bodies (20) loosely arranged within the hollow casing (17)so that water and steam may flow around them, and electrical heatingmeans connected to said hollow casing for electrically heating said heataccumulating bodies (20).